Improve error resilience and handling

Timeouts are now handled correctly and the Runner automatically
creates the execution environment if it could not be found in
Poseidon. The runner is deleted locally if Poseidon returns
a bad request error.

app/controllers/submissions_controller.rb

@@ -150,6 +150,10 @@ class SubmissionsController < ApplicationController
     end
 
     socket.on :exit do |exit_code|
+      # As this is sometimes called before the timeout is handled, we must not close the
+      # socket to the user here. The socket will be closed after handling the timeout.
+      next if exit_code == Runner::Connection::TIMEOUT_EXIT_STATUS
+
       EventMachine.stop_event_loop
       if @output.empty?
         tubesock.send_data JSON.dump({cmd: :write, stream: :stdout, data: "#{t('exercises.implement.no_output', timestamp: l(Time.zone.now, format: :short))}\n"})